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Despite the growing popularity of services like Foursquare — and Facebook's freshly-launched Places — location-based "check-in" sites are still poised to explode. In fact, a new study pegs their usage at around 4 percent of online adults. So, here's one idea to get people interested: save them money.

So, for those of you who haven't jumped on board yet, a check-in on Places or Foursquare or the like essentially allows you to broadcast to your buddies where you are. Whether you're using it to just let people know or to show them where to meet up, the obvious benefit to businesses is that it's free advertising on a social network. On Foursquare, "mayors" are even appointed — users who have checkedin at a place frequently.

To give folks a little added incentive to use the service, Facebook is partnering up with a bunch of different companies, universities and the like to offer discounts to those who check in. It's worth noting that Foursquare does this as well, though only for the mayors. It sounds like through Facebook Deals this would be more of a coupon-style service rather than a bonus to people who use the site the most.

While this sounds potentially awesome for us, it could be pretty crushing for similar sites already in place. After all, Facebook does have an enormous user base, which we imagine is an easy sell for companies. The one limiting factor right now could be that the virtual coupons are only available on an iPhone, though we imagine that'll change quick.